1. Field of the Invention
The present invention relates to a channel coding and particularly relates to an improved method and an apparatus for encoding binary data in such a way that the low-frequency component of encoded data is suppressed. The present invention further relates to an optical-disk recording method and an apparatus therefor.
2. Description of the Related Art
For many data-transmission systems and data-recording systems, suppression of low-frequency components in encoded data is important is essential for aiding a decoder in reproducing original data in a noisy environment and avoiding cross coupling with the components of other systems, such as a servo system.
A common strategy used for achieving such a spectral-constraint encoding is to add redundant information to a data word for performing the encoding such that the data word can be equivalently encoded into at least two substitution-information data signals. By selecting substitution information offering a low DC-component, a low-frequency spectrum of the encoded data can be effectively suppressed.
However, the quality of the low-frequency component suppression depends greatly on the amount of the redundant information used for achieving the DC-component suppression and a strategy for selecting a predetermined substitution-encoder mapping method from among a plurality of substitution-encoder mapping methods. Where the amount of the redundant information increases, the quality of the DC-component suppression improves and the encoding efficiency decreases. Therefore, it is important to select an effective strategy for selecting the predetermined substitution-encoder mapping method for designing a system that adds constraints on the DC component of encoded data and that maintains high encoding efficiency at low cost.
This document illustrates a method for improving the selection strategy commonly used for encoders and an apparatus therefor. Accordingly, it becomes possible to improve the quality of DC-component suppression without decreasing the encoding efficiency.
Hitherto, a plurality of methods has been introduced for adding redundant bit data to encoded data so as to suppress the DC component of the encoded data.
For example, in an 8/14 modulation (EFM) encoding method used for a compact-disk (CD) system, an 8-bit data-to-14-bit data translation table maps a sequence of data bytes to code words. The generated translation table satisfies a minimum run-length constraint and ensures that two binary xe2x80x9c1xe2x80x9d symbols during the encoding process are separated by at least two binary symbols xe2x80x9c0xe2x80x9d and that a maximum run-length constraint ensures that no binary sequence including ten successive xe2x80x9c0xe2x80x9d symbols or more occurs in the encoded data.
The minimum run-length constraint depends on the smallest feature size of the physical pit structure of a CD. The maximum run-length constraint is necessary for reliable clock reproduction during decoding process.
A sequence of three margin bits is inserted between each pair of code words. The values of the margin bits can be freely determined as long as the above-described run-length constraints are satisfied.
The freedom of determining the values of the margin bits is limited for the DC-component suppression. After selecting the margin bits, the run-length-encoded data including the margin bits are NRZI (non-return to zero inverse) modulated, so that each xe2x80x9c1xe2x80x9d symbol in the encoded data bit sequence is mapped to a bit transition in the modulation bit sequence.
For determining the values of the margin bits so that DC components are reduced, a running digital sum (RDS) value is determined to be the difference between the number of binary xe2x80x9c1xe2x80x9d symbols and the number of binary xe2x80x9c0xe2x80x9d symbols over the modulation bit sequence. The data to be encoded are presented to the encoder as 8-bit words in a sequential order. For each data word to be encoded, the margin bits are selected such that the RDS value approaches zero as much as possible and the number of xe2x80x9c1xe2x80x9d symbols and the number of xe2x80x9c0xe2x80x9d symbols in the modulation bit sequence are balanced.
Another example for using redundant information for reducing a DC component is an EFM+ encoding method used for a digital versatile disk (DVD) system. The EFM+ encoding method is an improved modification of the EFM encoding method. In the EFM+ encoding method, a data word of 8 bits is mapped to a code by using an 8 bit-to-16 bit state-dependent translation table.
Unlike the EFM encoding method, according to the EFM+ encoding method, there are no margin bits used for the DC-component control and code word concatenation. However, each data word can be encoded in another way by using a substitution table.
However, the selection strategy of the EFM+ encoding method for the DC-component suppression is essentially the same as that used in the EFM encoding method for each valid encoding substitution information, wherein an RDS value closest to zero is selected.
A third example of the DC-free encoding method is an 8/14 modulation parity preserving (EFMPP) encoding method proposed by Philips Corporation. According to this EFMPP encoding method, a data word of 8 bits is mapped to a code by using an 8/15 mapping table, as in the case of the EFM+ encoding. Since the EFMPP encoding method is unique and not used in general, there is nothing to show a substitution code for a data word. For achieving the DC-component control, the data bit sequence is therefore interleaved with a DC control bit sequence before the encoding is actually performed. Accordingly, the redundancy required for suppressing the DC component is provided.
Both the EFM encoding method and the EFM+ encoding method use the same strategy, that is to say, the RDS value is calculated for calculating the value of a DC-control bit. Further, the RDS value closest to zero is selected.
As has been described above, the inventors of the present invention conclude that there are different techniques for adding redundant information to encoded data, such as:
(1) Adding redundant information to a modulation bit sequence by using margin bits or substitution-encoder mapping, and
(2) Adding redundant information by inserting control bits into a bit data sequence before encoding.
However, in a particular encoding method such as the EFM encoding method, a DC-component suppression algorithm and an encoding translation method are tightly and architecturally integrated, and the DC-component suppression method and the encoding translation are conceptually separated from each other.
Therefore, the present invention does not depend on a particular encoding method such as the EFM encoding method used for the CD system or the EFM+ encoding method used for the DVD system. Rather, the present invention allows the encoder to select data for code mapping so as to achieve the DC-component suppression. By implementing a novel selection strategy for selecting substitution data for the code mapping, the low-frequency component of encoded data is suppressed.
In the above-described encoding methods, both the transition from the EFM encoding method to the EFM+ encoding method and the transition from the EFM+ encoding method to the EFMPP encoding method achieve high encoding efficiency. However, this high encoding efficiency is obtained at the sacrifice of the quality of the DC-component suppression. Therefore, there is a need for a better DC selection strategy that does not decrease the encoding efficiency.
Another example DC-free encoding method is the EFM combi-code (EFMCC) encoding method proposed by Philips Corporation. In the EFMCC method encoding, a data word of 8 bits is mapped to a code mostly by using an 8-to-15 main-code mapping method similar to that of the EFM+ encoding method. However, at predetermined word intervals, 8-to-17 substitution-code mapping is performed, which offers a choice between two code words. A subsequent state in a finite state machine (FSM) of one of these two code words is the same as that of the other. However, the parity of one of these two code words is opposite to that of the other, so that DC-component control is achieved. In this manner, the redundancy required for suppressing the DC component is provided. As in the case of the EFM+ encoding method, the same strategy for determining a suitable method for selecting a substitution code can be used through calculating the RDS value for each case and selecting a method that minimizes the RDS value. The EFMCC encoding can also be slightly modified for allowing the use of additional stochastic substitution information can be used, where a predetermined digital pattern appears in an encoded code sequence.
Recently, other determination algorithms have been introduced. For example, code-word selection is performed for minimizing the variance of the RDS, rather than the magnitude thereof. In another example, look-ahead encoding method is used. These algorithms have been introduced considering not only the effect of selecting substitution information for immediate surrounding code-words, but also a cumulative effect obtained by determining a predetermined number of future possible determination methods. However, the look-ahead algorithm was originally designed for the case where determination points occur at predetermined fixed intervals, as in the case of the insertion points of the substitution-code words used in the EFMCC encoding method. As a result, subsequent addition of possible stochastic substitution codes in the EFMCC encoding suggested an algorithm by which the look-ahead encoding was performed with only regular substitution-codes as determination points, with the additional stochastic substitutions being performed based only on local (partial) benefits on each branch extension as the look-ahead decision-tree was propagated. Because some of the stochastic substitutions can depend on a choice of a substitution code in the previous word, different branch extensions could receive different substitution patterns. The determination whether or not a particular substitution code should be used was based on its effect on the RDS value (or the RDS variance) of the current word and also the next word. While this technique offers some improvement, it does not take advantage of the stochastic substitution in an optimal manner, because it considers only their local benefits, rather than their effect on the look-ahead tree as a whole.
An object of the present invention is to provide an encoding method for achieving good DC suppression without decreasing the encoding efficiency and an apparatus therefor.
Another object of the present invention is to provide an optical-disk recording method using the above-described encoding method and the apparatus and an optical-disk recording apparatus.
According to a first aspect of the present invention, there is provided a method for encoding binary data. This methods includes the steps of performing encoder mapping for converting at least one data word having N data bit(s) into at least one binary modulation word having M data bit(s), wherein redundant information is provided so that a predetermined data word can be translated into a plurality of modulation words, and selecting a predetermined modulation word from among the plurality of modulation words obtained through the encoder mapping step, according to a look-ahead determination criterion method.
The look-ahead determination criterion method includes the steps of recording a current data word to be modulated and a predetermined number of future data words in either an original form or in a pre-encoded form, making a path search for evaluating a running digital sum (RDS) for a combinational tree whose space is expanded by a multiplier of the modulation words generated by performing the encoder mapping for the recorded current and future data words, and making a determination to select an encoding mapping method suitable for the current data word to be encoded by selecting a path nearest to bounds of the RDS of the path search.
A low-frequency component of a resulting binary modulation sequence is suppressed so that the value of a first-order spectrum becomes null at a frequency of 0 Hz.
According to a second aspect of the present invention, there is provided an encoding apparatus for encoding binary data. This encoding apparatus comprises an encoder mapping device for converting at least one data word having N data bit(s) into at least one binary modulation word having M data bit(s) and providing redundant information so as to translate a predetermined data word into a plurality of modulation words. This encoding apparatus further comprises a look-ahead determination criterion generator and a selector for selecting a predetermined modulation word from among the plurality of modulation words obtained through the encoder mapping device according to a criterion generated by the look-ahead determination criterion generator.
The look-ahead determination criterion generator includes a memory for,recording a current data word to be modulated and a predetermined number of future data words in either an original form or in a pre-encoded form. The look-ahead determination criterion generator further includes a path searcher for evaluating a running digital sum (RDS) for a combinational tree whose space is expanded by the plurality of modulation words generated through the encoder mapping performed for the recorded current data word and future data words. The look-ahead determination criterion generator further includes a determination device for selecting an encoding mapping method suitable for the current data word to be encoded by determining a path nearest to bounds of the RDS of the path search.
A low-frequency component of a resulting binary modulation sequence is suppressed so that the value of a first-order spectrum becomes null at a frequency of 0 Hz.
By using the above-described encoding method and the encoding apparatus, a sequence including binary source data is processed so that a sequence including binary DC-free modulation data is generated.
According to a third aspect of the present invention, there is provided an optical-disk recording method for recording an information signal onto an optical-disk recording medium by performing on/off modulation for continual laser light in a predetermined cycle based on a resulting encoded signal obtained through a predetermined encoding method. This encoding method includes the steps of performing encoder mapping for converting at least one data word having N data bit(s) into at least one binary modulation word having M data bit(s), wherein redundant information is provided so that a predetermined data word can be translated into a plurality of modulation words, and selecting a predetermined modulation word from among the plurality of modulation words obtained through the encoder mapping step according to a look-ahead determination criterion method.
This look-ahead determination criterion method includes the steps of recording a current data word to be modulated and a predetermined number of future data words in either an original form or in a pre-encoded form, making a path search for evaluating a running digital sum (RDS) for a combinational tree whose space is expanded by the plurality of modulation words generated by performing the encoder mapping for the recorded current and future data words, and making a determination to select an encoding mapping method suitable for the current data word to be encoded by selecting a path nearest to bounds of the RDS of the path search.
The step of making the determination includes the step of selecting an encoder mapping method suitable for the current data word to be encoded by determining a path that is nearest to the bounds of the RDS and that is nearest to the bounds of a running sum of the RDS.
As a result, a low-frequency component of a resulting binary modulation sequence is effectively suppressed so that the value of a second-order spectrum becomes null at the frequency of 0 Hz.
According to a fourth aspect of the present invention, there is provided an optical-disk recording apparatus comprising a laser-light source for emitting continual laser light, a modulator for modulating the continual laser light from the laser-light source and irradiating an optical-disk recording medium with the modulated laser light and an encoding device for encoding an information signal to be recorded onto the optical-disk recording medium into an on-off modulation signal and outputting the encoded on-off modulation signal to the modulator.
This encoding device includes an encoder mapping device for converting at least one data word having N data bit(s) into at least one binary modulation word having M data bit(s) and providing redundant information so as to translate a predetermined data word into a plurality of modulation words. This encoder further includes a look-ahead determination criterion generator, and a selector for selecting a predetermined modulation word from among the plurality of modulation words obtained through the encoder mapping device according to a criterion generated by the look-ahead determination criterion generator.
The look-ahead determination criterion generator includes a memory for recording a current data word to be modulated and a predetermined number of future data words in either an original form or in a pre-encoded form. The look-ahead determination criterion generator further includes a path searcher for evaluating a running digital sum (RDS) for a combinational tree whose space is expanded by the plurality of modulation words generated through the encoder mapping performed for the recorded current data word and future data words. The look-ahead determination criterion generator further includes a determination device for selecting an encoding mapping method suitable for the current data word to be encoded by determining a path nearest to bounds of the RDS of the path search.
The determination device has a selector for selecting an encoder mapping method suitable for the current data word to be encoded by determining a path that is nearest to bounds of the RDS and that is nearest to bounds of a running sum of the RDS.
A low-frequency component of a resulting binary modulation sequence is effectively suppressed so that the value of a second-order spectrum becomes null at a frequency of 0 Hz.
According to a fifth aspect of the present invention, there is provided a method for encoding binary data. This method includes the steps of performing encoder mapping for converting at least one data word having N data bit(s) into at least one binary modulation word having M data bit(s), wherein redundant information is provided so that a predetermined data word can be translated into a plurality of modulation words by using a regular substitution code and a stochastic substitution code, detecting a position of a word for which the regular substitution code and the stochastic substitution code are used, and selecting a predetermined modulation word from among the plurality of modulation words obtained through the encoder mapping step, according to a look-ahead determination criterion method.
The selection step includes the steps of storing a current data segment to be encoded with a variable length and a future data segment with a variable length in either an original form or in a pre-encoded form, and evaluating a determination criterion for each path search in a combinational tree whose space is expanded by the plurality of modulation words. The modulation words are generated by performing the encoder mapping for the current data segment and the future data segment that are stored in the memory. The selection step further includes the step of making a determination to select an encoding mapping method suitable for the current data segment to be encoded by the determining a path with a best value the determination criterion for the path search. As a result, a low-frequency component of a resulting binary modulation sequence is effectively suppressed.
According to a sixth aspect of the present invention, there is provided an encoding apparatus for encoding binary data. This encoding apparatus comprises an encoding that converts at least one data word having N data bits(s) into at least one binary modulation word having M data bit(s) and that provides redundant information for translating a predetermined data word into a plurality of modulation words by using a regular substitution code and a stochastic substitution code. This encoding appataratus further comprises a detector for detecting a position of a word for which the regular substitution code and the stochastic substitution code are used, and a selector for selecting a predetermined modulation word from among the plurality of modulation words, which is obtained by the encoder, according to a look-ahead determination criterion.
This selector includes a memory for storing a current data word to be with a variable length and a future data segment with a variable length in either an original form or in a pre-encoded form and an evalution device for evaluting a determination criterion for each path search in a combinational tree whose space is expanded by the plurality of modulation words. The modulation words are generated by performing the encoder mapping for the current data segment and the future data segment that are stored in the memory. This selector further includes a determination apparatus for making a determination to select an encoding mapping method suitable for the current data segment to be encoded by determining a path with a best value of the determination criterion for the path search.
Accordingly, a low-frequency component of a resulting binary modulation sequence is effectively suppressed.
According to a seventh aspect of the present invention, there is provided an optical-disk recording apparatus. This optical-disk recording apparatus comprises a laser-light source for emitting continual laser light and a modulator for modulating the continual laser light from the laser-light source and irradiating an optical-disk recording medium with the modulated laser light. This optical-disk recording apparatus further comprises an encoding device for encoding an information signal to be recorded onto the optical-disk recording medium into an on-off modulation signal and outputting the encoded on-off modulation signal to the modulator.
This encoding device includes an encoder for converting at least one data word having N data bit(s) into at least one binary modulation word having M data bit(s) and providing redundant information so as to translate a predetermined data word into a plurality of modulation words by using a regular substitution code and a stochastic substitution code. This encoding device further includes a detector for detecting a position of a word for which the regular substitution code and the stochastic substitution code are used and a selector for selecting a predetermined modulation word from among the plurality of modulation words, which is obtained by the encoder, according to a look-ahead determination criterion.
This selector includes a memory for storing a current data segment to be encoded with a variable length and a future data segment with a variable length in either an original form or in a pre-encoded form and an evaluation device for evaluating a determination criterion for each path search in a combinational tree whose space is expanded by the plurality of modulation words. This selector further includes the modulation words being generated by performing the encoder mapping for the current data segment and the future data segment that are stored in the memory, and a determination apparatus for making a determination to select an encoding mapping method suitable for the current data segment to be encoded by determining a path with a best value of the determination criterion for the path search.
Accordingly, a low-frequency component of a resulting binary modulation sequence is effectively suppressed.
The present invention has the following features:
(1) The encoding method and the apparatus therefor according to the present invention comprise at least one functional step and at least one apparatus, that is, an encoding mapping step and an apparatus for mapping a source data sequence to a modulation data sequence and selecting an effective method of valid source-to-modulation data mapping from among a plurality of valid source-to-modulation data mapping methods, and
(2) A look-ahead DC controlling method and an apparatus therefor according to the present invention for selecting a suitable method from among a plurality of substitution source-to-modulation data mapping methods so as to suppress the DC component of final modulation data.
The present invention is achieved by improving the above-described related art. The strategies of the present invention for selecting a suitable method for controlling a DC component and an apparatus therefor achieve better DC-component suppression.
The selection strategies of the present invention are shown as below:
(a) A look-ahead DC control method and an apparatus therefor that select a substitution encoding method by incorporating a data block to be encoded and a fixed number of future data words into determination, and
(b) A determination criterion on the basis of the RDS (or variance) or the modification thereof and the summation of the RDS (or variance) or a bound on the modification thereof.
The present invention does not modify the encoder mapping method, but improves the DC-selection strategy of the encoding A, which is modified by the present invention, resulting in an improved encoding method A+. In this case, there is no need to modify the decoder side for decoding encoded data encoded by using the improved encoding method A+. Consequently, good DC-component suppression is achieved. Therefore, it is possible to apply the present invention to a currently-used recording technology such as a CD system. In this case, the present invention increases the quality and reliability of a data storage disk by reducing the DC component of encoded data and eliminates the need of modifying the player of a user.